Lesson 2: I can estimate multi-digit
products by rounding factors to a
basic fact and using place value
patterns.
5th Grade Module 2 – Lesson 2
Concept Development
We are going to learn how to use estimation to help us find a product.
Let’s figure out about how many students we have in our school.
First, let’s use the number of students we have in our class to help us.
How many students are in our class?
Then, let’s think how many classrooms we have in our school.
How many classrooms?
Estimates can help us
understand a reasonable size
If we have 22 students in our class and we have 33 classrooms. What could I do
to find a number of
that is
to the actual when
number of students
aclose
product
we in our school?
In order to “estimate”
the totalthe
number
of students we can round
multiply
original
each number so it will be easier to multiply mentally.
numbers.
Let’s round 22 to 20 and let’s round 33 to 30.
We can now multiply 20 by 20
5th Grade Module 2 – Lesson 2
Let’s practice how we can use
rounding to estimate the product.
About
456 x 42 ≈______
20,000
How can I round these factors to estimate the product?
Remember, we want to be able to multiply mentally.
Which is the easier way to solve this?
460 x 40
or
What would the estimate be?
1. Solve in unit form:
5 hundreds x 4 tens
= 20 thousands
500 x 40
2. Solve in standard form:
(5 x 100) x (4 x 10) =
20 x 1,000 =
20,000
5th Grade Module 2 – Lesson 2
Let’s estimate the product of two more problems…
200,000
4,560 x 42 ≈ ______
5000 x 40 = 200,000
(5 x 4) x (1000 x 10)
20 x 10,000
Remember to think
which rounded value
would be the easiest
to mentally multiply!
2,000,000
4,560 x 420 ≈ ______
5000 x 400 = 2,000,000
(5 x 4) x (1000 x 100)
20 x 100,000
5th Grade Module 2 – Lesson 2
Problems 5-7
Round the factors to estimate the product:
1,320 x 88 ≈ ______
Let’s share some of your thinking.
Option 1:
1,000 x 90 = 90,000
Option 2:
1,300 x 90 =
(13 x 9) x (100x 10)
117 x 1,000
117,000
What if we change it to:
13,205 x 880 ≈ ______
Let’s test ourWhat
predictions.
Round
the estimate
is different
fromand
thefind
problem
above? product.
about
our estimate?
10,000What
x 900 ≈do you think will change
Was our
prediction
accurate?
(1 x 9) x (10,000 x 100)
9 x 1,000,000
9,000,000
Yes! 9 million is 100 times as
large as 90,000!
5th Grade Module 2 – Lesson 2
Work with your partner to estimate the following problems.
3,120 x 880
31,200 x 880
Volunteers to show their answers:
5th Grade Module 2 – Lesson 2
Get Ready to Complete the
Problem Set on Your Own!
Complete Pages 2.A.23 & 2.A.24
You will have 10-15 minutes to work.
Try your Best!
5th Grade Module 1 – Lesson 5
5th Grade Module 1 – Lesson 5
EXIT
TICKET
5th Grade Module 1 – Lesson 5